Polymers containing hydroxamate groups have been known to be particularly useful for the flocculation of red muds produced in the Bayer process, see U.S. Pat. No. 4,767,540. U.S. Pat. No. 6,020,418 describes the preparation of these hydroxamated polymers as water-in-oil microdispersions. The hydroxamated polymer microdispersions are high molecular weight polymers in which the hydroxamate functional group is introduced by the reaction of a suitable reactive functional group on the polymer, usually an amide from acrylamide monomer unit in the polymer, with a hydroxylamine salt, usually hydroxylamine sulfate, at a high pH. Since this known process requires the use of a base to convert the hydroxylamine salt to the free hydroxylamine that is reactive with the reactive functional group on the polymer and sufficient water to dissolve the by-product inorganic salt, the resulting hydroxamated polymer microdispersion is fairly dilute compared to the starting backbone polymer dispersion. There is a need to manufacture a more concentrated hydroxamated polymer dispersion, which could result in a more efficient manufacturing process with a higher throughput for the polymer. This would also mean that less material would need to be handled and shipped for the required polymer needed in the application. Another problem with the current technology using hydroxylamine salt is that the dispersion contains significant amounts of by-product salt, which is usually sodium sulfate or sodium chloride. Indeed, per hydroxamate group present on the polymer, an equivalent of inorganic salt is formed. The inorganic salts are undesirable impurities in the Bayer process which contribute to the scaling problem in these plants and contribute to the corrosivity of the Bayer liquor. These impurities build up as the liquor is recycled and can retard the recovery of alumina from solution, act as templates for the crystallization of aluminosilicates which coprecipitate with the alumina trihydrate, contribute to the deposition of scale on equipment (particularly in heaters contributing to reduced evaporation efficiencies), all of this leading to a requirement for increased plant size to accommodate the increased circulating load.
The present invention provides a method for preparing microdispersions of hydroxamated polymers and microdispersions of hydroxamated polymers which overcome these problems.